The present invention relates to a process for the electrochemical fluorination of organic materials.
Fluorochemicals (e.g., fluorinated and perfluorinated organic compounds) are commercially valuable and useful chemical materials. Fluorochemicals can exhibit various useful properties, e.g., they may be inert, nonpolar, hydrophobic, oleophobic, etc. As such, fluorochemicals can be useful in a wide variety of applications. They can be useful as oil, water, and stain resistant chemicals; they can be useful as refrigerants and heat exchange agents; or as solvents and cleaning agents. Due to the versatility of fluorochemicals, and a consequent strong demand for these materials, there is a continuing need in the fluorochemical industry for new and improved methods of preparing fluorochemicals.
One well-known industrial process for preparing fluorochemical compounds is the electrochemical fluorination process commercialized initially in the 1950""s by the 3M Company. This process, often referred to as Simons fluorination or electrochemical fluorination (ECF), is a method by which electric current is passed through an electrolyte solution containing a mixture of liquid anhydrous hydrogen fluoride and an organic compound intended to be fluorinated (the xe2x80x9csubstratexe2x80x9d). Generally it is taught that the Simons process is practiced with a constant current passed through the electrolyte; i.e., a constant voltage and constant current flow. See for example W. V. Childs, et al., Anodic Fluorination in Organic Electrochemistry, H. Lund and M. Baiser eds., Marcel Dekker Inc., New York, 1991. The current passing through the electrolyte causes one or more of the hydrogens of the substrate to be replaced by fluorine.
The Simons process of electrochemical fluorination, although commercially useful, includes aspects that might desirably be improved upon. For example, the Simons process requires a significant amount of electrical energy passing through the electrolyte solution. Much electrical energy is effectively used to fluorinate the substrate, but a certain amount of this electrical energy converts to heat energy that must necessarily be carried away from the electrochemical fluorination cell as wasted energy, and adds to the overall cost of operating the process. It would be desirable to reduce the amount of electrical energy that is wasted as dissipated heat energy in the Simons process, and thereby reduce the overall cost of electricity needed to operate this process.
Also, the conventional Simons process often includes the use of conductivity additives to allow the passage of current through the electrolyte solution. See for example J. Burdon and J. C. Tatlow, The Electrochemical Process for the Synthesis of Fluoro-Organic Compounds, Advances in Fluorine Chemistry, edited by M. Stacey et al., volume 1 p. 129 (1960). Conductivity additives can cause undesired results when used in the Simons process. Conductivity additives, for example, can interfere with the fluorination of the substrate, either by causing increased corrosion of the anode, or by themselves being consumed or fluorinated in the fluorination reaction. This can reduce the overall yield of the desired fluorinated product, and in many ways can increase the costs of the fluorination operation. Therefore, it would be desirable to reduce or even substantially eliminate the need for conductivity additives.
Finally, the Simons process can be difficult to maintain at steady state for extended periods of time because high resistance by-product films and tars can tend to accumulate on electrodes of the fluorination cell, specifically, at the anode. In normal operation, the accumulation of films and tars on the anode causes increased resistance of the electrochemical cell, and an upward drift in cell voltage. The problem can become more serious and lead to the condition referred to as xe2x80x9ccurrent blocking,xe2x80x9d which is manifested as a permanent increase of resistance and loss of conductivity within the cell. To correct current blocking often requires shut-down of the apparatus for cleaning. It would therefore be desirable to prevent increases in the resistance of a fluorination cell that can lead to loss of conductivity within the cell, and the permanent condition of current blocking.
The present invention relates to a process for the electrochemical fluorination of chemical compounds such as those containing organic moieties. In the process, a reaction solution is provided that comprises an organic substrate and hydrogen fluoride. An electric potential (voltage) is established across the reaction solution causing an electric current to pass through the reaction solution, and thereby causing fluorination of the organic substrate. In the method, the electric current is periodically and regularly interrupted, i.e., the current flows at a first current level, identified as an elevated current level, and is periodically interrupted to flow at a reduced current level. In the invention, the current is interrupted in such a manner that the resistance of the electrochemical fluorination (ECF) cell operated under the conditions of the present invention is lower than the resistance of the cell operated without interruption of the current.
Interrupting the electric current during fluorination offers a number of advantages over conventional electrochemical fluorination methods. As stated, the current can be interrupted in such a manner that the resistance of the fluorination cell will be reduced compared to the resistance of the same fluorination cell operating with uninterrupted current. The reduced cell resistance resulting from interrupted current in turn results in a lower cell voltage being required to achieve fluorination at constant current; i.e., the voltage between the anode and the cathode with interrupted electric current can be lower relative to the voltage of the same cell, operated with the same amount of current, wherein the current is interrupted. At the same time, because the cell resistance is comparatively lower, the amount of wasted heat energy created during the fluorination process is also reduced, reducing or eliminating the need to remove wasted heat energy from the fluorination cell. The achievement of a lower operating voltage allows stable cell operation at higher current densities, which in turn allows more product to be produced in a given time period, and extended production runs (e.g., days, weeks, etc.) without significant interruptions. At production scale, reduced cell voltage, reduced cell resistance, and increased current, result in a more efficient process with higher production rates of a fluorinated product, often at a lower cost. Furthermore, interrupted current can reduce or eliminate the need for conductivity additives in an electrochemical fluorination process. This can reduce corrosion of the electrodes within the fluorination cell, reduce the amount of energy and raw materials wasted due to fluorination of the conductivity additives themselves, and reduce unwanted by-products. In some cases, interrupted electric current increases the selectivity of the fluorination reaction resulting in higher yields and reduced by-products. All of these identified improvements advantageously reduce overall operation costs for the electrochemical fluorination process.
An aspect of the present invention relates to a process for fluorinating a substrate using an electrochemical fluorination cell. The process includes the steps of: (1) providing a substrate comprising at least one carbon-bonded hydrogen; (2) preparing a reaction solution comprising the substrate and hydrogen fluoride; (3) passing electric current through the reaction solution sufficient to cause replacement of one or more hydrogens of the substrate with fluorine. In the process, the electric current is interrupted through a cycle defined by current levels comprising an elevated current and a reduced current, and in such a manner that the resistance of the cell operated with interrupted current is lower than the resistance of the cell operated with uninterrupted current.
Another aspect of the present invention relates to a method for perfluorinating a substrate according to the above method.
A particular aspect of the present invention relates to a process for fluorinating an alkane substrate using an electrochemical fluorination cell. The process includes the steps of: (1) providing an alkane substrate comprising at least one carbon-bonded hydrogen; (2) preparing a reaction solution comprising the alkane substrate and hydrogen fluoride; (3) passing electric current through the reaction solution sufficient to cause replacement of one or more hydrogens of the alkane substrate with fluorine. In the process, the electric current is interrupted through a cycle defined by current levels comprising an elevated current and a reduced current, and in such a manner that the resistance of the cell operated with interrupted current is lower than the resistance of the cell operated with uninterrupted current.
Yet another particular aspect of the invention relates to a process for electrochemical fluorination including the steps of: providing a substrate having at least one carbon-bonded hydrogen; providing a fluorochemical in which the substrate is soluble; providing an electrochemical fluorination cell; providing hydrogen fluoride; introducing the substrate to the fluorochemical so that the substrate dissolves into the fluorochemical; introducing the hydrogen fluoride to the electrochemical fluorination cell; introducing the fluorochemical with substrate dissolved therein, to the fluorochemical cell, at a temperature that is below the temperature of the hydrogen fluoride; and passing electric current through the cell sufficient to cause replacement of one or more hydrogens of the substrate with fluorine.
As used within the present description, and in reference to an electrochemical fluorination process:
xe2x80x9cFluorinatedxe2x80x9d refers to chemical compounds having at least one carbon-bonded hydrogen replaced by a fluorine, and specifically includes perfluorinated compounds. xe2x80x9cPerfluorinatedxe2x80x9d compounds refers to chemical compounds in which essentially all carbon-bonded hydrogens have been replaced by fluorines, although typically some residual hydride will be present in a perfluorinated composition; e.g., preferably less than 1 milligram hydride per gram perfluorinated product.
xe2x80x9cUninterrupted currentxe2x80x9d refers to the electric current flowing through an electrochemical fluorination cell, wherein the current is substantially constant; i.e., not substantially varied, and specifically, not periodically interrupted as described in the following description.